1. Field of the Invention
This disclosure relates to a flexible substrate for a display device, and a display device using the same. More particularly, the disclosure relates to a flexible substrate for display devices. The flexible substrate comprises a plastic layer that comprises a woven glass fiber, a metal film coated on either both side surfaces or one side surface of the plastic layer, and an overcoating layer formed on one side surface of the metal film, wherein the overcoating layer is on the side of the metal film opposite from the plastic layer. The flexible substrate is thin, and has a small thermal expansion coefficient, excellent flexibility, heat resistance, gas permeability resistance and also displays resistance of moisture.
2. Description of the Related Art
It is desirable for display devices such as a liquid crystal displays (LCD) and an organic electro luminescent displays (OLED) to display a high performance such as long-term reliability coupled with light weight. In addition, it is desirable for such display devices to permit a designer flexibility in design of such displays. Design flexibility includes freedom of shape (e.g., producing curvilinear displays, irregular shaped displays), freedom to produce devices of both large and small sizes with substantial weight variations. The growth in portable electronic equipment such as portable computers and cellular phones has promoted studies on flexible substrates using a plastic in lieu of glass substrates, which are heavy, large in thickness, easily broken, and difficult to bend. Materials such as polyethylene terephthalate, polyethylene naphthalate, polycarbonate, polyimide, polyethylene sulfonate, polyoxymethylene, polyacrylate or a fiber reinforced plastic can be used for the various flexible substrates.
However, those plastic substrates have disadvantages. For example, their heat resistance, solvent resistance, gas permeability resistance and water vapor transfer resistance are inferior to those of glass substrates. Especially, when using the plastic material as a substrate for a display device, inferiority in the gas permeability resistance and moisture blocking property (hereinafter, referred to as ‘gas barrier property’) of plastic substrates can be an obstacle to its utilization.
One method of imparting gas barrier properties to the plastic substrate comprises coating a polyvinylalcohol-based resin layer on the plastic substrate. However, the gas barrier layer comprising the polyvinylalcohol-based resin layer has disadvantages. Although the gas barrier property is good in a low humidity atmosphere of 50% or lower relative humidity, the gas barrier property deteriorates at a humidity higher than 50% relative humidity. Additionally, polyacrylonitrile, polyvinylchloride can be used as a material for a gas barrier layer. However, these materials have problems such as difficult to handle, environmental issues associated with applying them to the plastic substrate and the like.
Another method of imparting gas barrier properties to the plastic substrate comprises coating a metal oxide on the plastic substrate. In this method, the gas barrier property of the resulting coated material remains the same at different humidities. However, other properties such as mechanical properties are deteriorated. For example, when the metal oxide layer is thick enough to provide the desired gas barrier property, the metal oxide layer cracks easily. Moreover, in the case of silicon oxide, which is used widely as the metal oxide layer, the gas barrier layer located in the outer most layer of the substrate and is easily eroded by an aqueous alkali or alkaline solution.
Another problem associated with the plastic substrate is that the thermal expansion coefficient of the plastic substrate is larger when compared to glass substrates. This large thermal expansion coefficient can lead to separation of the glass portion from the plastic portion of a composite glass-plastic substrate when there are large temperature changes.
Therefore, the development of a new shape flexible substrate, which is lightweight and flexible, and devoid of the problems listed above is desirable.